The invention concerns an acousto-optic device particularly suitable for deflecting light beams and in apparatus for processing of information wherein the diffraction of light on acoustic waves in the optically anisotropic medium a single crystal of univalent mercury halide.
Existing acousto-optic devices for deflecting light beams and for optical processing of information operate on the principle of the diffraction of the light wave by the application of an acoustic wave induced in a suitable acousto-optic medium by a piezoelectric transducer. By changing the frequency of the acoustic wave the deflection angle of the light beam is changed. By changing the amplitude of the acoustic wave the intensity of the deflected beam is controlled. The diffraction of the light beam on application of the acoustic wave can also result in a change of the polarization of the light wave. Depending upon the application in systems used for deflecting the light beams--e.g.: deflectors--and in systems used for processing of information, the most important parameters of the acousto-optic device are the diffraction efficiency, i.e. the ratio between the intensities of the deflected and incident beam, and the product of the time constant .tau. of the device with the width .DELTA.f of the frequency band. This bandwidth is determined on one hand by the electric and acoustic properties of the piezoelectric transducer and on the other hand by the interaction bandwidth of the acoustic and light wave. An extremely large bandwidth of the acousto-optic interaction can be attained in an optically anisotropic medium by making use of the so-called abnormal diffraction, in which the polarization of the deflected light beam changes. In an optically uniaxial media the transverse acoustic wave is introduced, as a rule, in a direction parallel or perpendicular to the optical axis, whereas the direction of the incident beam is chosen so as to have the deflected beam emerging perpendicular to the optical axis.
In using the crystal types known so far, the described arrangement leads to rather high acoustic frequencies in the gigahertz range and the diffraction efficiency is low.
In the crystal of paratellurite, strong rotation of the polarization plane of light can be utilized for the construction of an acousto-optic device with abnormal diffraction and high diffraction efficiency. The operating frequency of such acousto-optic device is relatively low, in the range of tens of megahertz, depending on the wavelength of light employed. In this case, however, the incident light must have approximately circular polarization.
An acousto-optic deflector utilizing the abnormal diffraction in a rotated tellurium dioxide crystal is also known. The acoustic wave in the tellurium dioxide crystal propagates in the direction inclined at angle of 6.degree. from the [110] axis in the (110) plane, with the [110] direction of vibrations. This arrangement retains the high efficiency of interaction and compensates for the drop of the diffraction efficiency in the middle of the frequency band. A disadvantage of this deflector is that the group velocity direction of the acoustic wave is inclined from the wave normal at a large angle, nominally 51.3.degree.. Consequently, an extremely large crystal volume is required for the construction of such a deflector. Besides, deflectors made from the tellurium dioxide crystals cannot be used in the infrared spectral range beyond 5 .mu.m. Another disadvantage is the relatively high price for the tellurium dioxide single crystals of the required dimensions and quality. In addition, still another disadvantage of the acousto-optic device made from tellurium dioxide consists in the fact that the acousto-optic quality factor M.sub.2, which determines the diffraction efficiency for the diffraction by longitudinal waves, is rather small--about 1/30 of the value for the diffraction by transverse waves.
A acousto-optic device made from a single crystal of univalent mercury halide, is described by Dobrzhanskii et al. in the CSSR Author's Certificate No. 170 007, which has high value of the acousto-optic quality factor M.sub.2 for both longitudinal and transverse waves, and which in addition transmits the radiation even in the infrared spectral rage with wavelength larger than 5 .mu.m. A disadvantage of this device, however, is that, due to the low propagation velocity of the acoustic wave, sufficient frequency bandwidth can be obtained only by using a piezoelectric transducer of very small dimensions, whereby the requirements on the acoustic power density generated by the transducer are very severe.
There was also reported an acousto-optic device made from a single crystal of univalent mercury halide, pure or mixed, by C. Barta et al. in U.S. Application Ser. No. 968,930 filed Dec. 13, 1978; in which the single crystal is treated in the following way: at least one of the first pair of its opposite faces is formed perpendicular to the direction which is inclined from the direction of the optical axis of the crystal [001] about the crystallographic direction [HKO] by an angle whose value lies from 0.1.degree. to 15.0.degree., the other crystal face is provided with the source of the acoustic wave being perpendicular to the above-mentioned face. This solution has resulted in significant advantages over the arrangements known till then; however, it did not suppress parasitic diffraction of light in unwanted directions, resulting therefore in a decrease of the diffraction efficiency within the frequency band of the acousto-optic device.
The above-mentioned shortcomings are avoided in the acousto-optic device according to the invention.